Electrical method and apparatus for the recovery of oil

ABSTRACT

Two well bores extend from the surface into the oil bearing formation defining a producing well and an electrode well. Electrodes in each well, contacting the formation, are connected to a unidirectional current voltage source at the surface through conductive tubing or pipe in the respective well bores to produce a unidirectional voltage gradient between the electrodes, with the producing well poled to be the cathode. Additionally, an alternating current voltage source is connected between the producing well electrode and another conductive path extending from the surface to the formation, to effect the flow of alternating current through the formation adjacent to the producing well to heat the formation.

United States Patent Gill s41 ELECTRICAL METHOD AND APPARATUS FOR THERECOVERY OF OIL [72] Inventor: William G. Gill, Corpus Christi, Tex.

[73] Assignee: The Eleetrothermic Co., Corpus Christi,

Tex.

[22] Filed: Nov. 13, 1969 [211 Appl. No.: 876,462

[ 51 Feb. 15, 1972 Primary Examiner-[an A. Calvert Attorney-Giles C.Clegg, Jr. and Peter .1. Murphy [57] ABSTRACT Two well bores extend fromthe surface into the oil bearing formation defining a producing well andan electrode well.

[52] U.S.Cl ..166/248, 166/52, 166/60 Electrodes in each well,contacting the fomiation, are con- [51] Int. Cl ..E2lb 43/16 nected to aunidirectional current voltage source at the sur- [58] Field of Search..166/248, 302, 303, 52, 60 f through conductive tubing or pipe in therespective well bores to produce a unidirectional voltage gradientbetween 56] References Cited the electrodes, with the producing wellpoled to be the cathode. Additionally, an alternating current voltagesource is UNITED STATES PATENTS connected between the producing wellelectrode and another conductive path extending from the surface to theformation, 2,799,641 7/ 1957 Bell ..166/248 to effect the flow ofalternating current through the formation 2,801,090 7/l957 HPYCI et a1.166/248 X adjacent to the producing we" to heat the formation 2,818,11812/1957 Dixon ..l66/248 3,103,975 9/1963 Hanson ..166/248 X 15 Claims, 3Drawing Figures I I I AC 1 I SOURCE 7 lo PAIENTEBHB 15 I972 SHEEI 1 OF 2wOmDOm lNl/E/VTOR WILLIAM G. GILL f @09 ATTORNEYS PATENTEDFEB 15 IanSHEET 2' BF 2 m T L N L w I IN, G 6 Mv M X L w I u w m N OE mm momDowmomzom ATTORNEYS BACKGROUND OF THE INVENTION This invention relates toan apparatus and method for elec- 5 trically stimulating the productionof oil from a subsurface formation, and more particularly to suchapparatus and method utilizing the effect of electro-osmotic pressure.This invention is concerned with the movement of oil through a reservoirformation including rock or sand, where flow of the oil under the extantdriving forces to a well bore has reduced to the point where it is nolonger economically producible.

It is known that the movement of oil through a fonnation is adverselyeffected by the presence of water in the formation, and it is also knownthat the effective permeability of the formation to the flow of oilvaries somewhat inversely with the percentage of water saturation in theformation. Accordingly, if the percentage of water saturation in theformation can be reduced, or if the percentage of oil saturation can beincreased, the flow of oil within the formation may be increased to asignificant degree. It is particularly desirable to improve thepercentage of oil-to-water saturation in the area of the formationimmediately adjacent to the producing well, since the greatest hydraulicpressure gradient involved in moving fluids from the formation into thewell occurs within this area.

It is an object of this invention to provide an improved apparatus andmethod employing electrical means for stimulating the flow of oil from aformation into a producing well.

It is another object of this invention to provide such an improvedapparatus and method employing electro-osmotic means.

It is a further object of this invention to provide such an improvedapparatus and method employing a combination of electro-osmotic andelectric heating means.

The apparatus and method according to the invention include theprovision of adjacent well bores extending from the surface to the oilproducing formation, defining a producing welI bore and an electrodewell bore. Electrodes are placed in each well bore in electrical contactwith the formation. A source of unidirectional supply voltage isconnected between the electrodes through suitable conductive pipes orrods placed in the well bores to cause the flow of direct currentthrough the formation originating and terminating within the formation.The conductive pipes are effectively insulated from the walls of theboreholes above the electrodes to assure maximum potential differenceand current flow within the producing formation and to reduceelectrolytic corrosion of the pipe. The insulation may be effected bystrings of insulating casing extending fro a substantial length abovethe electrodes. The producing well is poled to be the cathode of theunidirectional current circuit. To further stimulate oil flow, theformation in the area of the producing well may be heated by means ofalternatin g current carried through the producing well electrode and anadditional conductive path which may be provided in the producing wellbore or in an adjacent well bore. A source of alternating current supplyvoltage connected between the electrode and the additional conductivepath directs alternating current through the formation adjacent to theborehole, the current being carried through connate water in theformation to heat the formation and the oil in order to reduce itsviscosity.

DRAWINGS The novel features of the invention, as well as additionalobjects and advantages thereof, will be understood more fully from thefollowing description when read in connection with the accompanyingdrawings, in which:

FIG. 1 is a diagrammatic illustration of an earth formation including aproducing well and an electrode well embodying one form of theinvention;

FIG. 2 is a diagrammatic illustration of an earth formation including aproducing well and an electrode well embodying another form of theinvention; and

FIG. 2A is a fragmentary illustration of a modification of the form ofFIG. 2.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to FIG. I, a'producing well bore 10 and an electrode well bore 40 extend from thesurface into an oil bearing formation 12 lying between the overburden l1and an underburden 13.

The well bore is cased from a point adjacent to the top of the formation12 to the surface, with the portion of the bore within the formationdefining an open hole completion. The casing includes a lower portion,consisting of an insulating casing 15 fabricated of fiberglass forexample, which extends up wardly from the formation for a substantialdistance into the overburden 11. The remaining upper casing portion 16may be a conventional metal casing, fabricated preferably of a goodconductive metal. A metallic screen 17 is provided in the lower openportion of the well bore below the casing and is secured in the boreholeand mechanically coupled to the insulating casing 15 by a packer 18. Thescreen 17 is preferably seated on a bottom plug 19 of insulating cementor epoxy, for reasons to be described.

A string of conventional tubing 21 defines the production tubing for thewell bore 10, extending from the bottom of the bore hole to the surface;and this tubing is preferably of a good electrically conductive metal todefine a low resistance conductive path from the surface to the bottomof the borehole.

An electrode 22' for providing good electrical contact with theformation 12 is defined, for example, by a mass of conductive particles23 carried to the bottom of the well bore and urged into an annularcavity 24 formed in the walls of the well bore below the casing 15. Theconductive particles 23 may consist, for example, of metallic or carbonpellets or metallic pellets coated with carbon. The conductive particlesare packed or urged into the cavity by conventional techniques beforethe placement of the screen 17; and the screen is then placed to retainthe particles within the cavity and within the annulus between thescreen and the bore, the screen then defining a part of the electrode22. The electrode is connected to the lower end of the conductive tubing21 by means of a metallic centralizer 25 which is fixed to the lower endof the string of tubing and has bands which are bowed outwardly intoengagement with the inner surface of the metallic screen. The bands ofthe centralizer may be coated with carbon to improve electrical contactbetween the centralizer and the screen.

For reasons which will be described, it is important that the conductivetubing 21 be well insulated from the formation in the area of theelectrode 22, and from the conductive casing 16 which extends to thesurface. For this purpose, the insulating casing 15 is provided; and afurther insulating effect may be provided by cementing the well borethroughout the extent of the insulating casing with an insulating cementor epoxy 26.

To insulate the tubing from the conductive casing 16, a string ofinsulating casing 27 is provided extending from the surface to thescreen 17; and the annulus between the insulating tubing and theinsulating casing at the lower end is preferably sealed by the packer 18which secures the sleeve in position.

The electrode well includes the well bore 40 which extends from thesurface into the formation 12, the well bore 40 also being cased with acasing which extends from the surface downward to a point short of thebottom of the well bore. An electrode 42 is defined in the bottom of theelectrode well bore 40; and the casing includes a lower insulatingportion 43 which extends upward from the electrode for a substantialdistance, into the overburden 11 as viewed in the drawing, while theupper portion 44 of the casing may be of any conventional materialincluding an electrically conductive material.

A low resistance conductive path is provided in the electrode well boreby a string of conductive tubing or rod 45 which extends from thesurface to the bottom of the well bore.

This tubing or rod is preferably fabricated of a metal having goodconductive characteristics and is sealed relative to the lower end ofthe insulating casing 43 by a packer 46.

The .portion of the conductive tubingor rod, which extends below thepacker 46, may consist of a carbon rod 47 joined to the tubing 45 in anysuitable manner. The lower end of the well bore 40 may be filled with amass of conductive particles, such as metallic or carbon pellets ormetallic pellets coated with carbon, which surround and engage thecarbon rod 47. To insure good electrical contact with the formation 12and to increase the diameter of the electrode, the particles are urgedinto an annular cavity or notch 49 extending laterally from the walls ofthe well bore. The electrode 42 then is defined by the conductiveparticles 48 which are in electrical contact with the carbon rod 47.

As with the producing well bore 10, it is desirable to effec- 'tivelyinsulate the conductive path to the electrode 42 for a substantialdistance above the electrode. This is accomplished in part by theinsulating casing 43; and additionally such casing may be cemented inthe borehole by an insulating cement or epoxy 50. The conductive tubing45 can be insulated from the walls of the well bore 40 remote from theelectrode 42 by providing insulating spacers 51 in the annulus betweenthe tubing 45 and the conductive casing 44. If desired, the entirestring of casing, that is both the portions 43 and 44, may be fabricatedof aninsulating material such as fiberglass.

At the surface, a source of unidirectional current voltage 55,preferably a pulsating direct current voltage, is connected between theproducing well and the electrode well, one terminal of the source beingconnected through a conductor 56 to the conductive tubing 21 in theproducing well bore, and another terminal of this source being connectedthrough a conductor 57 to the conductive tubing or rod 45 of theelectrode well bore. As indicated in the drawing, the negative side ofthe direct current voltage source is connected to the producing well;and accordingly the electrode 22 of the electrode well is poled to bethe cathode, and the electrode 42 of the producingwell is poled to bethe anode. It will be seen,

between the electrodes 22 and 42, a potential difference or gradient isestablished through the formation causing a unidirectional flow ofcurrent. It has been found from tests that oil in the formation movestoward the cathode and that the water in the formation moves toward theanode. This results in a reduction of the water saturation andaccompanying increase in oil saturation in the area of the cathodeelectrode 22 at the producing well, and a corresponding increase inwater saturation and decrease in oil saturation in the area of the anodeelectrode 42. This increase of oil saturation in the area of theproducing well, coupled with the increased permeability of the formationto the flow of oil resulting from the reduction of the water saturation,results in the increased flow of oil into the producing well. Thisphenomenon of movement of fluids through a porous solid under theinfluence of an electrical potential difference is referred to aselectro-osmosis.

Where the producing formation 12 is a generally horizontal stratum, asin the diagram of F IG. 1, it may be advantageous to place the producingwell electrode 22 at a higher elevation than the electrode 42 in orderto have the additional benefit of the gravity flow of the connate waterrelative to the oil. The flow under electro-osmotic pressure will occur,however, independently of the relative elevation of the electrodes.

To further stimulate the flow of oil from the formation adjacent to theproducing well electrode into the well, the formation is heated by asecond electric circuit. As illustrated in FIG. 1, the electric circuitfor heating the formation includes an alternating current voltage source60 provided at the surface and having one terminal connected by aconductor 61 to the upper end of the conductive tubing 21, and anotherterminal connected by a conductor 62 to the upper end of the conductivecasing 16 of the producing well. This provides an alternating currentpath through the conductor 61, the tubing 21, the electrode 22, andupward through a portion of the formation l2 and of the overburden 11 tothe conductive casing 16, and through the conductor 62 back to thesource.

The alternating current flowing through the formation is conductedprincipally by the connate or other water in the formation, therebyheating the formation and the oil within the formation. The principaleffect of the heating is to reduce the viscosity of the oil within theformation to further stimulate the flow of the oil into the producingwell.

While some of the elements of the above described direct current circuitand alternating current circuit are common to both circuits, such as theelectrode 22, the circuits function independently of each other. Onecircuit defines a closed loop system for unidirectional current and theother circuit defines a closed loop system for alternating current, sothat there is no interference of the circuits with each other or theirintended functions.

With the flow of unidirectional current through the formation betweenthe electrodes 22 and 42, there will be dissipation or consumption ofthe electrodes due to electrolysis; and accordingly, it is desirablethatthe electrodes which are in contact with the formation 12 be formed in amanner that they are replacable or replenishable. Electrodes formed ofthe described conductive particles are well suited to this purpose. Therate of electrode dissipation is a function of he current density; andaccordingly it is desirable to establish electrodes in contact with theformation which are sufficiently large to minimize, to the extentpossible, the effect of electrolysis. The described electrodes, whichare radially enlarged by extension into the annular cavities, are againwell suited to this purpose.

It is desirable to prevent dissipation of the screen 21, by the actionof electrolysis; and for this reason the screen is seated on theinsulating bottom plug 19, and the annulus between the screen and thewalls of the well bore are completely filled with the electrodeparticles 23. The screen 17 then is electrically isolated from theformation so that the electrode dissipation is confined to theconductive particles 23. For the same reason, the carbon rod 47 of theelectrode well terminates short of the bottom of the well bore and iscompletely surrounded by the conductive particles 48. Similarly, toprevent any dissipation of the conductive casing 16 of the producingwell, the insulation between the electrode 22 and the casing 16 providedby the insulated casing 15 and the insulating cement 26 extends for asubstantial distance above the electrode and, preferably, into theoverburden 11. This minimizes the possibility of any direct current flowthrough the formation other than through the electrode 22. Similarly,the insulation of the electrode well, defined by the insulating casing43 and the cement 50, extends for a substantial distance between theelectrode and the conductive casing 44, preferably into the overburdenformation 11. The insulation of the producing well bore defined by theinsulating casing 15 and cement 26 provides the additional function ofassuring a flow path of substantial length in the formation 12, betweenthe electrode 22 and the casing 16 to produce the desired resistanceheating in the formation due to the flow of alternating current.

FIG. 2 is a diagrammatic illustration of an alternative system accordingto the invention including a producing well and an electrode well. Inthis system, the producing well is defined by a well bore extending fromthe surface into the oil bearing formation 12, and having an enlargedbottom portion 71 within the formation. A bottom plug 72 is formed inthe bottom of the bore hole from insulating epoxy or insulating cement,for example. The well bore 70 is cased with a casing 74 which extendsfrom the surface into the enlarged well bore portion 71, the casing inthis system being fabricated of a conductive metal and being providedwith an external insulating layer or coating 75. An annular cavity 76 isformed in the wall of the enlarged well bore portion 71; and theenlarged bore and the cavity are filled with conductive particles orpellets 77 to define an electrode 78 which has a diameter substantiallygreater than that of the well bore 70. The bore portion 71 is completelyfilled with the conductive particles 77 so that these particles surroundand engage both the outer and inner surfaces of the casing portion whichextends into the bore portion 71. A conductive screen 79 is then placedin the bottom of the borehole resting on the bottom plug 72 and extendsupwardly within the casing 74 concentric therewith to define an annularspace 80 between the screen and casing. The screen is physically coupledto the casing by means of a packer 81 placed at the upper end of thisannular space 80, the packer and screen then confining and retaining theconductive particles in the bore portion 71 outside of the screen.

The interior conductive walls of the casing 74 then are in electricalcontact with the conductive particles confined in the annular space 80to provide the conductive path between the casing and the electrode 78.Since the conductive particles surround the lower end of the casing, thecasing is not in direct contact with the formation so that dissipationof the casing due to electrolysis will be inhibited.

A string of conductive tubing 83 extends from the surface to the screen79, defining the production tubing for the well and also defining asecond conductive path to the electrode 78. The lower end of the tubing83 is electrically connected to the screen 79 by means of a centralizer84 fixed to the tubing and engaging the inner walls of the screen.

With this described arrangement, the conductive casing .74 and thetubing 83 provide electrically parallel conductive paths through thewell bore 70 to the electrode 88. This may be particularly desirable toincrease the effective conductor area to obviate losses due to excessiveand unnecessary heating of the conductor. Additionally, this arrangementpermits the carrying of larger currents to the electrode, which may beparticularly desirable in this system where the conductive casing andtubing are employed as conductors in two independent close loopcircuits.

While the casing 74 defines a conductive path, the insulating coating 75also defines a complete insulation of the path from the walls of thewell bore. The continuity of the insulating coating throughout thelength of the well bore may be assured by applying insulating materialto the joints as the casing is set and rapidly curing the material bytechniques which are known in the art.

The electrode well is defined by a well bore 90 which also includes anenlarged portion 91 at the bottom of the bore. The well bore is casedwith a string of casing 92 which extends from the surface downwardly andpartially into the enlarged well bore portion 91, the casing 92.alsobeing fabricated of a conductive metal and provide with an interiorinsulating coating or layer 93. During the setting of the casing thecontinuity of the insulating layer may again be assured by coating thejoints with additional insulation material.

An annular cavity 94 isprovided in the walls of the enlarged well boreportion 91; and an electrode 95 is formed by filling the enlarged wellbore portion and the cavity with a mass of conductive metal or carbonparticles 96. These particles are packed around the lower end of thecasing 92 which extends into the bore 91, engaging both the exterior andinterior surfaces of the casing to isolate the conductive interiorsurface from the walls of the bore. The mass of particles also extendsupwardly within the casing for a sufficient distance to assure goodelectrical contact between the particles and the casing wall. A stringof conductive tubing or rod 97 is provided in the bore extending fromthe surface and into the mass of conductive particles 96, but spacedfrom the bottom of the well bore, to provide a second conductive pathfrom the surface to the electrode 95. The conductive particles may beretained in place by means of a packer 98 placed to seal the annularspace between the casing 92 and the tubing 97. For the electrode wellthen the casing and tubing define electrically parallel paths from thesurface to the electrode, providing the same advantages as the parallelconductive paths for the production well.

To provide the above described electro-osmotic pressure between theproducing well 70and the electrode well 90, a source of unidirectionalcurrent voltage 100 is provided at the surface, the negative terminalbeing connected by means of conductors 101 and 102 tothe production welltubing 83 and casing 74 respectively, and the positive terminal beingconnected by means of conductors 103-and 104 to the electrode welltubing 97 and casing 92 respectively. There is provided then, a closedloop electrical system for providing a unidirectional potential gradientbetween the producing well electrode 78, which is poled to be thecathode, and the electrode well electrode 95, which is poled to be theanode.

There is also provided at the surface a source of alternating currentvoltage 105 having one terminal connected by means of conductors 106 and107 to the producing well, tubing 83 and casing 74 respectively, andhaving another terminal connected by means of conductors 108 and 109 tothe electrode well tubing 97 and casing 92 respectively. This defines asecond closed loop electrical system for effecting the flow ofalternating current through the formation 12 between the producing wellelectrode 78 and the electrode well electrode 95. While the conductivepaths from the surface to the electrodes are common for each closed loopelectrical system, the systems function independently and withoutinterference from each other.

In the system illustrated in FIG. 2, the producing formation 12.is astrata which is inclined relative to the horizontal; and the producingwell electrode 78 is preferably located in a portion of the strata whichis at higher elevation to obtain any benefit of the gravity flow ofwater from the producing well toward the electrode well.

A method for stimulating the recovery of oil from an oil bearingformation, which may be practiced with the above described apparatus,may include steps which will now be described. At least two spaced wellbores are provided extending from the surface to the producingformation, at least one well bore defining a producing well and anotherwell bore defining an electrode well. Electrodes are establishedin eachof the bores in contact with the producing formation, and preferablyextending laterally from the bore to define laterally enlargedelectrodes. A first closed loop electrical system is provided forcausing the flow of unidirectional current through the formation betweenthe electrodes; this system including a source of unidirectional supplyvoltage at the surface and suitable conductor means provided in the wellbores which connect the voltage source and the electrodes whichpreferably include conductive tubing or casing in the well bores.Through this first closed loop system. direct current is caused to flowbetween the electrodes through the formation to effect the flowof oilinthe formation toward the producing well which is poled to be thecathode, and to effect the flow of water toward the electrode well whichis poled to be the anode.

A second closed loop electrical system is provided for causing'the flowof alternating current through the formation, at least adjacent to theproducing well to heat the formation and thereby reduce the viscosity ofthe oil in the formation. This alternating current closed loopsystem mayinclude the same electrodes and the same conductive tubing or casingdefining the conductors through the well bores, which are employed inthe unidirectional system. In. this case, both the alternating currentand the direct current will flow through the formation between theelectrodes of the producing and electrode well bores. However, since thetwo systems are each closedloop systems, they function independently ofeach other and without interferencev from each other.

Alternatively, an alternating current system may be provided to includethe electrode and conductors within the producing well, which are commonto the unidirectional current system, and aseparate electrode andconductor which may be provided in the producing well here or adjacentthereto for causing the flow of alternating current to be confined to anarea surrounding the producing well. FIG. 2A illustrates such analternative arrangement of the system of H6. 2 wherein the separate:electrode is provided by a surface casing 111 which encloses the casing74 for a relatively short distance beneath the ground surface. FlG. 2Ais a fragmentary illustration of the upper portion of the producing wellbore 70 of FIG. 2 including the conductive casing 74 with the insulatingcoating 75 and the conductive tubing 83, which function in the mannerdescribed and define portions of both closed loop electrical systems asdescribed. In this modification, however, one terminal of thealternating current source 105 is connected to both the tubing 83 andthe casing 74 by means of the conductors 106 and 107, as alreadydescribed, while the other terminal of the source 105 is connected tothe upper end of the conductive surface casing 111 by means of theconductor 112. With the arrangement the advantages of theparallel'conductive paths through the producing well bore aremaintained, and additionally the flow of alternating current through theformation is confined to an area adjacent to the producing well bore.

While the above described apparatus and method have been described withparticular reference to two well bores defining one producing well andone electrode well, the method and apparatus may be practiced as wellwith a combination of well bores defining, for example, a singleproducing well with a multiplicity of electrode wells, a singleelectrode well with a multiplicity of producing wells, or a multiplicityof both producing wells and electrode wells.

What has been described are apparatus and a method employing differentelectrical techniques and phenomena acting on an oil bearing formationto stimulate or improve the flow of oil within the formation to aproducing well bore. The apparatus required for the practice of thesemethods is relatively inexpensive as compared with apparatus requiredfor other known techniques of secondary oil recovery. Examples of othersuitable apparatus are disclosed in my copending applications Ser. Nos;752,1 l2 filed July 10, 1968, now abandoned, and 767,917 filed Sept. 30,1968, now U.S. Pat. No. 3,507,330, assigned to the assignee of thisapplication.

From tests which have been conducted, it appears that the area of theformation which responds to the electro-osmotic effect, is in proportionof the sizes of the electrodes. Accordingly, it is desirable toestablish electrodes which have a large effective diameter, that is,much larger than the diameter of the respective bore holes. Electrodesas described herein may be established to have any desired diameter. Thearea of the formation which is effectively heated by the above describedalternating current heating apparatus is also related to the size of theelectrode; and accordingly,.an electrode of enlarged diameter may bedesirable in the heating circuit.

The efficiency with which the electro-osmotic effect or the electricheating effect are provided in the formation is dependent upon producingthe electric power at electrodes, either within or adjacent to theformation, with minimum losses between the electrodes and the voltagesources. Accordingly, it is most desirable to provide good conductivepaths between the voltage sources and the electrodes to obviate anyunnecessary voltage losses; and also to insulate against any extraneouscurrent paths which would carry the flow of current outside of thedesired paths within the formation. The described systems are examplesof efficient apparatus, according to the invention for practicing themethod of the invention.

What is claimed is:

1. Apparatus for the recovery of oil from a subsurface oil bearingformation penetrated by at least two well bores including at least oneproducing well and at least one electrode well comprising: I

a first closed loop,electrical system for causing the flow of Iunidirectional current through the fonnation; and a second closed loopelectrical system for causing the flow of alternating current throughthe formation;

said unidirectional current .closed loop system comprises:

first and second electrode means, each positioned in said producing andelectrode wells respectively in electrical contact with the oil-bearingformation, with the electrode means being spaced from each other; firstand second conductor means connected respectively to said first andsecond electrode means and extending to the surface through therespective well bores; and a source of unidirectional voltage connectedat the surface between said conductor means to produce a unidirectionalpoten tial gradient between said first and second electrode means ofsaid unidirectional system, with the first electrode means poled to be acathode;

said alternating current closed loop system comprising: said firstelectrode means positioned in the well bore of the producing well inelectrical contact with the oil-bearing formation; a source ofalternating current voltage; and means including the first conductormeans positioned in the producing well for completing an electricalcircuit through the formation between the first electrode means and thesource of alternating current voltage.

2. Apparatus as set forth in claim 1 wherein said second electrode meansand said second conductor means for said unidirectional current systemcomprise respectively the second electrode means and conductor means forsaid alternating current system.

3. Apparatus as set forth in claim 1 wherein each of said electrodemeans comprises amass of conductive particles disposed in the bottom ofthe respective well bores; wherein the conductor means for each of thewell bores extends into the respective mass of conductive particles tomake electrical contact therewith;

and including insulating means for insulating each of said conductormeans from the walls of respective well bores; said insulating meansextending into the mass of conductive particles to confine the currentflow to a path through said conductive particles.

4. Apparatus as set forth in claim 1 wherein each of said electrodemeans comprises a mass of conductive particles which maintains contactwith the formation and is consumable.

5. Apparatus as set forth in claim 1 wherein each of said electrodemeans for said unidirectional current system comprises a mass ofconductive particles disposed in an annular cavity extending laterallyfrom the respective well bores.

6. Apparatus as set forth in claim 1 wherein said first conductor meansof the producing well comprises a string of conductive pipe extendingfrom the surface to said first electrode.

7. Apparatus as set forth in claim 1 wherein said first conductor meansof the producing well comprises a string of conductive casing and astring of conductive tubing electrically connected in parallel betweensaid first electrode and the surface; and insulating means comprising aninsulating coating provided on the outer wall of said conductive casing.

8. Apparatusfor the recovery of oil from a subsurface oilbearingformation penetrated by twowell bores comprising;

first electrode means positioned in a producing well bore in electricalcontact with the oil-bearing formation; first conductor means in saidproducing well bore extending from said first electrode means to thesurface; insulating means for insulating said first conductor means fromthe walls of said producing well bore above said first electrode means;

second electrode means positioned in an electrode well bore inelectrical contact with the formation; second conductor means in saidelectrode well bore extending from said second electrode means to thesurface; insulating means for insulating said second conductor meansfrom the walls of the well bore above said second electrode means;

third electrode means positioned in spaced relation to said firstelectrode means; third conductor means extending from said thirdelectrode means to the surface;

a source of unidirectional voltage connected at the surface between saidfirst and second conductor means to provide a unidirectional potentialgradient through the formation between said first and second electrodemeans, with said first electrode means poled to be a cathode;

a source of alternating current supply voltage connected between saidfirst conductor means and said third conductor means to cause the flowof alternating current between said electrodes and through saidformation adjacent to said producing well.

9. Apparatus as set forth in claim 8 wherein said first conductor meanscomprises a string of conductive tubing; wherein said third electrodemeans and third conductor means comprise a string of conductive casingplaced in said producing well bore extending from the surface towardsaid formation; insulating means insulating said string of conductivecasing from said electrode to provide a conductive path of substantiallength through said formation between said first and third electrodemeans; and wherein said first named insulating means comprises a stringof insulating casing disposed between said conductive tubing and saidconductive casing.

10. Apparatus as set forth in claim 8 wherein said first conductor meanscomprises a string of conductive casing extending from the firstelectrode means to the surface; wherein said insulating means for saidconductive casing comprises a coating of insulating material provided onthe exterior surface of said conductive casing; and wherein said thirdelectrode means and said third conductor means comprise a string ofconductive surface casing enclosing the upper portion of said coatedconductive casing.

11. Apparatus for the recovery of oil from a subsurface oilbearingformation penetrated by two well bores comprising:

first consumable electrode means positioned in a producing well borecomprising a mass of conductive particles urged into a cavity in theformation extending laterally from the well bore;

a first string of electrically conductive pipe in the producing wellbore having its lower end connected to said first electrode and havingits upper end extending to the surface;

a first string of conductive casing positioned in said producing wellbore and provided with an insulating coating for insulating said firststring of conductive pipe and said first string of easing from the wallsof the well bore above the first electrode means;

means electrically connecting said first string of conductive pipe andsaid first string of conductive casing to define parallel electricalconductors in said producing well bore between said electrode and thesurface;

second consumable electrode means positioned in an electrode well borecomprising a mass of conductive particles urged into a cavity in theformation extending laterally from the well bore;

a second string of electrically conductive pipe in the electrode wellbore, having its lower end contacting said second electrode and havingits upper end extending to the surface;

a second string of conductive casing positioned in the electrode wellbore and provided with an insulating coating for insulating said secondstring of conductive pipe and said second string of casing from thewalls of the electrode well bore;

means electrically connecting said second conductive pipe and saidsecond string of conductive casing to define parallel electricalconductors in the said electrode well bore between said second electrodeand the surface; and

a source of unidirectional voltage connected at the surface between saidfirst and second string of conductive pipe to provide a unidirectionalpotential gradient between said first and second electrodes, with thefirst electrode in said producing well bore poled to be a cathode.

12. A method for recovering oil from a subsurface oil-bearing formationcomprising the steps:

electrode well; placlng first conductor means In said producing wellbore contacting said first electrode and extending to the surface;

placing second conductor means in said electrode well bore contactingsaid second electrode and extending to the surface;

connecting a source of unidirectional supply voltage between said firstand second conductor means, with said first conductor means poled to bea cathode;

insulating said first and second conductor means from the walls of thewell bores to cause the flow of unidirectional current between the firstand second electrode means through the earth to originate and terminatein the oilbearing formation;

and connecting a source of alternating current supply voltage betweensaid first conductor means at the surface and another electrode meansspaced apart from said first electrode means to cause the flow ofalternating current between said first and said another electrode meansthrough said formation in the area of said producing well to heat saidformation.

13. A method as set forth in claim 12 including placing surface casingin said producing well bore

1. Apparatus for the recovery of oil from a subsurface oil bearing formation penetrated by at least two well bores including at least one producing well and at least one electrode well comprising: a first closed loop electrical system for causing the flow of unidirectional current through the formation; and a second closed loop electrical system for causing the flow of alternating current through the formation; said unidirectional current closed loop system comprises: first and second electrode means, each positioned in said producing and electrode wells respectively in electrical contact with the oil-bearing formation, with the electrode means being spaced from each other; first and second conductor means connected respectively to said first and second electrode means and extending to the surface through the respective well bores; and a source of unidirectional voltage connected at the surface between said conductor means to produce a unidirectional potential gradient between said first and second electrode means of said unidirectional system, with the first electrode means poled to be a cathode; said alternating current closed loop system comprising: said first electrode means positioned in the well bore of the producing well in electrical contact with the oil-bearing formation; a source of alternating current voltage; and means including the first conductor means positioned in the producing well for completing an electrical circuit through the formation between the first electrode means and the source of alternating current voltage.
 2. Apparatus as set forth in claim 1 wherein said second electrode means and said second conductor means for said unidirectional current system comprise respectively the second electrode means and conductor means for said alternating current system.
 3. Apparatus aS set forth in claim 1 wherein each of said electrode means comprises a mass of conductive particles disposed in the bottom of the respective well bores; wherein the conductor means for each of the well bores extends into the respective mass of conductive particles to make electrical contact therewith; and including insulating means for insulating each of said conductor means from the walls of respective well bores; said insulating means extending into the mass of conductive particles to confine the current flow to a path through said conductive particles.
 4. Apparatus as set forth in claim 1 wherein each of said electrode means comprises a mass of conductive particles which maintains contact with the formation and is consumable.
 5. Apparatus as set forth in claim 1 wherein each of said electrode means for said unidirectional current system comprises a mass of conductive particles disposed in an annular cavity extending laterally from the respective well bores.
 6. Apparatus as set forth in claim 1 wherein said first conductor means of the producing well comprises a string of conductive pipe extending from the surface to said first electrode.
 7. Apparatus as set forth in claim 1 wherein said first conductor means of the producing well comprises a string of conductive casing and a string of conductive tubing electrically connected in parallel between said first electrode and the surface; and insulating means comprising an insulating coating provided on the outer wall of said conductive casing.
 8. Apparatus for the recovery of oil from a subsurface oil-bearing formation penetrated by two well bores comprising: first electrode means positioned in a producing well bore in electrical contact with the oil-bearing formation; first conductor means in said producing well bore extending from said first electrode means to the surface; insulating means for insulating said first conductor means from the walls of said producing well bore above said first electrode means; second electrode means positioned in an electrode well bore in electrical contact with the formation; second conductor means in said electrode well bore extending from said second electrode means to the surface; insulating means for insulating said second conductor means from the walls of the well bore above said second electrode means; third electrode means positioned in spaced relation to said first electrode means; third conductor means extending from said third electrode means to the surface; a source of unidirectional voltage connected at the surface between said first and second conductor means to provide a unidirectional potential gradient through the formation between said first and second electrode means, with said first electrode means poled to be a cathode; a source of alternating current supply voltage connected between said first conductor means and said third conductor means to cause the flow of alternating current between said electrodes and through said formation adjacent to said producing well.
 9. Apparatus as set forth in claim 8 wherein said first conductor means comprises a string of conductive tubing; wherein said third electrode means and third conductor means comprise a string of conductive casing placed in said producing well bore extending from the surface toward said formation; insulating means insulating said string of conductive casing from said electrode to provide a conductive path of substantial length through said formation between said first and third electrode means; and wherein said first named insulating means comprises a string of insulating casing disposed between said conductive tubing and said conductive casing.
 10. Apparatus as set forth in claim 8 wherein said first conductor means comprises a string of conductive casing extending from the first electrode means to the surface; wherein said insulating means for said conductive casing comprises a coating of insulating material provided on the exterior surfAce of said conductive casing; and wherein said third electrode means and said third conductor means comprise a string of conductive surface casing enclosing the upper portion of said coated conductive casing.
 11. Apparatus for the recovery of oil from a subsurface oil-bearing formation penetrated by two well bores comprising: first consumable electrode means positioned in a producing well bore comprising a mass of conductive particles urged into a cavity in the formation extending laterally from the well bore; a first string of electrically conductive pipe in the producing well bore having its lower end connected to said first electrode and having its upper end extending to the surface; a first string of conductive casing positioned in said producing well bore and provided with an insulating coating for insulating said first string of conductive pipe and said first string of casing from the walls of the well bore above the first electrode means; means electrically connecting said first string of conductive pipe and said first string of conductive casing to define parallel electrical conductors in said producing well bore between said electrode and the surface; second consumable electrode means positioned in an electrode well bore comprising a mass of conductive particles urged into a cavity in the formation extending laterally from the well bore; a second string of electrically conductive pipe in the electrode well bore, having its lower end contacting said second electrode and having its upper end extending to the surface; a second string of conductive casing positioned in the electrode well bore and provided with an insulating coating for insulating said second string of conductive pipe and said second string of casing from the walls of the electrode well bore; means electrically connecting said second conductive pipe and said second string of conductive casing to define parallel electrical conductors in the said electrode well bore between said second electrode and the surface; and a source of unidirectional voltage connected at the surface between said first and second string of conductive pipe to provide a unidirectional potential gradient between said first and second electrodes, with the first electrode in said producing well bore poled to be a cathode.
 12. A method for recovering oil from a subsurface oil-bearing formation comprising the steps: providing at least two spaced-apart well bores defining a producing well and an electrode well; establishing first electrode means in the formation at the producing well; establishing second electrode means in the formation at the electrode well; placing first conductor means in said producing well bore contacting said first electrode and extending to the surface; placing second conductor means in said electrode well bore contacting said second electrode and extending to the surface; connecting a source of unidirectional supply voltage between said first and second conductor means, with said first conductor means poled to be a cathode; insulating said first and second conductor means from the walls of the well bores to cause the flow of unidirectional current between the first and second electrode means through the earth to originate and terminate in the oil-bearing formation; and connecting a source of alternating current supply voltage between said first conductor means at the surface and another electrode means spaced apart from said first electrode means to cause the flow of alternating current between said first and said another electrode means through said formation in the area of said producing well to heat said formation.
 13. A method as set forth in claim 12 including placing surface casing in said producing well bore to define said another electrode means.
 14. A method as set forth in claim 13 wherein the source of alternating current supply voltage is connected between said first conductor means And said second conductor means, and said another electrode means is said second electrode means.
 15. A method as set forth in claim 12 including establishing said first and second electrode means to extend laterally from the respective well bores to define electrodes having effective diameters substantially greater than those of the respective well bores. 